Although glass containers are substantially impenetrable and provide products with long shelf life, they are heavy and expensive for manufacturing and transportation. Containers made of polymeric materials now replace glass containers in applications where traditionally glass containers were used. Plastic containers are less susceptible to breakage, less expensive to manufacture, and lighter and less expensive to ship. Used for packaging are plastic materials such as polyethylene terephthalate (PET) and high-density polyethylene (HDPE) in the form of bottles or other shapes having an opening at the top.
PET containers are used for liquids such as wine, soft drinks, etc. HDPE is used for packaging milk, water, juice, cosmetics, shampoo, etc. HDPE containers are more available for recycling than PET containers and serve a second life for the packaging of liquid laundry detergents, shampoo, conditioner, motor oil, etc.
However, glass properties such as chemical resistance and permeability are not attainable for plastics. Polymer-chain clearance of a plastic structure is less than 1 nm and, hence, cannot prevent penetration of low-molecular gases having molecules ranging in size from 0.3 to 0.4 nm. The walls of such packages are permeable in both directions in relation to gases such as oxygen, carbon dioxide, etc. The shelf life of liquids is limited, especially for soft drinks and other CO2-containing liquids. A long shelf life is required for carbonated beverages (soft drinks and beer), fruit juice, cosmetics, chemicals, and pharmaceuticals. Deterioration of liquids on the shelf, especially in hot weather, is caused by several factors: (1) oxygen, which is responsible for entering the container through the plastic wall from outside; (2) carbon dioxide, which escapes through the same container wall; and (3) low chemical resistance of a PET container to strong contents such as a carbonated soft drink or alcoholic beverages. Molecules of liquid absorbed at high temperature in hot weather or during microwave heating are combined with hydroxyl, thus grouping the polymer matrix and weakening the existing hydrogen bounds between the polymer molecules. As a result, interchange distances increase and create free volume, which facilitates the diffusion of oxygen and perhaps the diffusion of other gases as well. If a PET package contains a flavor compound (such as orange juice or apple juice), this compound causes swelling of the PET container, i.e., opening the structure and further increasing the specific free volume that leads to oxygen transport. Therefore, flavor absorption significantly increases oxygen permeability of PET.
One can expect a reduction in shelf life of oxygen-sensitive products because of higher oxygen permeability. At 23° C., 50% relative humidity (RH), and atmospheric pressure (oxygen at 0.21 atmosphere) outside a bottle, a 0.5-liter nominal-volume bottle formed from PET has an oxygen transmission rate of 0.126 cc/bottle/day. Under the same conditions, the same bottle formed from HDPE has an oxygen transmission rate of 8.47 cc/bottle/day. Flavoring ingredients of low-molecular organic compounds existing in drinks such as lemonade absorb the plastic material and thus deteriorate the quality of the drink. For these reasons, plastic containers are unsuitable for drinks, especially those with carbon dioxide, alcohol, and flavoring ingredients.
In order to prolong the shelf life of these liquid products, a better gas barrier is required. The barrier property of plastic containers can be improved by coating the inner walls of these containers with a transparent layer, e.g., quartz-like SiO2. The aforementioned barrier layer should remain after hot filling or pasteurization. Besides reducing the permeability of the containers, the layer that absorbs UV irradiation, which causes deterioration in the taste of wines and other beverages, is also included in this barrier. Such coating is provided by means of plasma-enhanced chemical gas-phase deposition (PECVD) of an organosilicon compound having an excess of oxygen.
The PECVD process is described, e.g., by J. Felts in U.S. Pat. No. 6,180,191 issued in 2001. A PECVD-applied silicon dioxide (SiO2) layer on the inside surface of a PET bottle prevents the ingress of oxygen and the egress of carbon dioxide that would affect the taste of the product and its shelf life. After deposition of a thin silicon oxide coating, the oxygen transmission rate is reduced to 0.076 cc/bottle/day.
The PECVD process first deposits a transparent adhesive layer of nanocrystalline SiO and then a colorless silicon oxide (SiOx) barrier layer having a thickness of 0.01 to 0.1 micron. The SiOx layer improves the oxygen-barrier properties of a bottle more than 10 times, and the SiO2 barrier, specifically, improves this property more than seven times. These barrier improvements remain after hot filling or pasteurization. In addition to the use of PECVD in the food and pharmaceutical industries, application of a PECVD barrier onto the inner surfaces of hollow objects may be used in automotive and piping industries wherein plastic materials such as HDPE are used to replace metals because of their excellent tensile strength and impact properties at temperatures as low as −50° C. and at temperatures as high as 70° C., which match the temperature range in fuel tanks and pipe lines. Since HDPE is low in weight and cost, it is competitive with steel. However, HDPE has one drawback, and this is permeation of fuels. In order to overcome this drawback, it is necessary to develop an improved barrier coating suitable for application onto the inner surfaces of HDPE tanks and pipes, especially those designed to contain gasoline, alcohol, or other toxic, corrosive, and health-hazardous materials. Moreover, the same coating system is supposed to serve as an inductive probe to provide quality control of the thickness, uniformity, and integrity of the barrier in the inner surface of the wall after the deposition process. The SiO2 coating has high optical transparency and a markedly improved barrier effect as well as greater tensile strength. Silicon dioxide is nontoxic and does not affect the recycling of PET and HDPE.
The inner container coating of SiO2 provides an excellent gas permeation barrier because of two important properties. First, the coating on the interior surface of the container is not subject to abrasion during shipment and handling when compared to the exterior surface of the container. Second, by forming the coating on the interior surface of the container, degradation of the product within the container from direct interactions between the product and the container is prevented.
Thus, there is a demand for a simple, inexpensive, and reliable process for application of barrier coatings onto the inner walls of polymeric containers. The process should have a fast cycle time to accommodate production demands and be suitable for integration into a bottle-molding production line, such as a Husky molding system with throughput of 15,500 bottles per hour. Further, the barrier coating should have good uniformity, and the barrier-coated polymeric container should be easy to recycle.
A plasma-enhanced chemical vapor deposition (PECVD) coating from a gaseous phase is well known and is used in the semiconductor industry to treat semiconductor wafers. However, a flat substrate such as a semiconductor wafer, which is an object of deposition, can be treated at high temperatures with application of a bias voltage, while in the case of plastic containers, the material of such containers has a low melting point that cannot withstand high temperatures. Plasma discharge is developed by an RF antenna introduced into the container together with a gas mixture and when the RF antenna is energized, this causes a plasma-chemical reaction that results in generation of silicon dioxide, which is deposited onto the inner walls of the containers in the form of a thin barrier layer of SiO2. The plasma-chemical reaction can be conducted between different silicon-containing gases such as silane or disilane and oxygen-containing gases such as nitrogen dioxide, nitrous oxide, etc. Because of the flammability and explosiveness of silanes, the above process requires special, expensive facilities in the semiconductor industry. The food industry prefers to conduct the processes under less expensive, unpretentious conditions with a safer organosilicon or siloxane and by conducting the plasma chemical-reaction with pure oxygen. The plasma-chemical reaction may also have safe-reaction byproducts, such as CO2 and water. Plasma discharge inside a container decomposes siloxane vapor and breaks off methyl groups. Further, the oxygen oxidizes the condensable siloxane backbone (Si—O—Si) resulting from the organosilicon decomposition, thereby forming a plasma-enhanced chemical vapor deposition (PECVD) thin film of silicon oxide (SiOx) on the interior surface of the container. Gaseous organosilicon is received, for example, from liquid tetraethylorthosilicate (TEOS). TEOS can be converted into vapor by using a direct liquid injection subsystem DL125-C (a product of MKS Company) that includes a vaporizer that evaporates the liquid into vapor for introducing it into the processing system. Byproducts (CO2 and water) are removed by means of a vacuum system through small holes provided in a bottle holder.
The pure SiO2 barrier, however, presents some disadvantages because it is brittle and can be torn during bending and squeezing. In order to enhance durability of the coating, a double-layer coating is preferred wherein the first thin layer is a layer of nanocrystalline SiO2 deposited on the plastic wall. This first layer blocks the porosity of plastic and simultaneously improves the adhesion to plastic of the next thick layer of amorphous SiO2 intended for contact with the liquid. This layer increases chemical resistance of the wall to aggressive species and simultaneously reinforces the barrier layer to prevent rupture of the film.
The methods and devices for generation of plasma used to form barrier coatings inside plastic containers are adopted from the sterilization processes inside the bottles described in the British Patent GB 1,098,693 (Menashi, et al., issued in 1968). Menashi describes a device for sterilization inner surfaces of plastic bottles by a method in which a central electrode is introduced into a bottle that is surrounded by an external electrode. Two electrodes form a coaxial system connected to a high-frequency current source. Argon (Ar), as a process gas with low potential of ionization, is introduced into the bottle through a hole in the central electrode in order to develop a capacitively coupled plasma (CCP) discharge. The device described in this patent is characterized by a high electric field, of the order of 450 V/cm, and a very weak current, of the order of a few milliamps at high RF power. The low current of the CCP discharge is caused by losses of RF power sustaining the discharge because 70% of this power is wasted by bias-current heating of the inner and outer electrodes, as well as the plastic between the electrodes. The CCP discharge is divided by the plastic wall on the useful discharge inside the bottle, the discharge providing deposition and parasite discharge between the outer electrode and the outer wall of the bottle. The parasite discharge consumes a valuable part of RF power. Only a small part of RF power sustains the inner discharge used for sterilization. The treatment time of sterilization is too long for application of this process in industry. Another disadvantage of such a method is sputtering of the electrodes in the CCP discharge by high-energy ions of argon and contamination of the inner surface of the container by material of the inner electrode.
In spite of such disadvantages, Thomas, et al (see U.S. Pat. No. 5,378,510 for “Methods and apparatus for depositing barrier coatings” issued in 1995) adopted the above-described geometry because of its simplicity. The authors of the above invention proposed to use the RF discharge to decompose process gas delivered through a gas inlet referred to as “adjacent axis conduit extending into hollow container. Decomposition of the process gas forms organosilicon vapor, which is deposited in the form of a barrier layer of SiOx onto the inner surface of the bottle, called ‘a hollow polymeric container’. RF power was applied to the outer electrode, called “an electrically conductive shell surrounding hollow container.”
U.S. Pat. No. 7,166,336 issued in 2000 to Mori, et al, and U.S. Pat. No. 6,180,191 issued in 2001 to Felts disclose the use of the same coaxial deposition system individually for each bottle with some differences in bottle evacuation procedures. The Felts process occurs in a vacuum chamber wherein the outer electrode is located adjacent to an exterior surface of the chamber, but Mori combines the coaxial deposition system with the vacuum chamber, while the outer electrode serves as a wall of the vacuum chamber that is individual for each bottle. The gas inlet in both systems is the same as proposed by Thomas, but the supply of gas is carried out through a plurality of small holes. The structure includes an immersed, grounded central electrode of the coaxial system and supplies the PECVD process with a gaseous precursor.
RF power is applied to an outer electrode located adjacent to an exterior surface of the chamber and to the inner electrode combined with the gas inlet. In “inverse” radial flow reactors, the gas inlet is at the center of the lower electrode, with the gas flow directed radially outward. The PECVD thin film, after decomposition, deposits onto the interior surface of the container. In the Thomas case, the bottle is rotated to enhance uniformity of the barrier layer. In the Felts case, the inner electrode is rotated by a magnetic drive in order to randomize the substrate position that faces the gas stream and to optimize uniformity of deposition. However, Mori, who reduced the clearance between the outer electrode and the outer wall of the container in order to reduce parasitic discharge from the bottle, has divided the outer electrode, which tightly envelops the container, into three parts: (1) a bottom portion of the electrode that is disposed along the bottom of the plastic container; (2) a body portion of the electrode that is disposed along the body of the plastic container; and (3) a shoulder portion that is located above the body portion enveloping the neck of the container. Resistive or capacitive elements are interposed between the outer electrodes to provide distribution of RF power and simultaneously to seal the outer electrode that serves as an individual vacuum chamber. An output terminal of the RF generator is connected only to the first portion of the outer electrode through a matching network. The aforementioned distribution of RF power makes it possible to provide varying plasma density at the bottom, middle, and neck of the container. This design provides uniformity in coating thickness on the inner surfaces of the bottom, body, and neck of the container, which are differently spaced from the inner electrode. Although the devices proposed by Mori, Thomas, and Felts generate coating films of different types (in Mori's case, these are diamond-like films, and in the Thomas and J. Felts cases, these are silicon dioxide films), the devices suffer from the same disadvantages that are inherent in CCP discharge, in general.
The main disadvantage of aforementioned processes and devices is that application of the CCP discharge to coat the inner surfaces of a container is carried out at a low-deposition rate limited by 10 nm/sec, a rate that significantly reduces throughput of a production line. On the other hand, lengthy treatment of plastic materials at high flux of thermal energy generated by electrodes softens the plastic to the extent that after reaching a critical point, a container can collapse. In order to increase the deposition rate, plasma density must be increased (e.g., by increasing pressure inside the container), and also RF power that sustains the discharge must be increased. On one hand, increase in pressure leads to breakdown of the space between electrodes by the arc between both electrodes, which damages the container. On the other hand, high RF power initiates corona discharge on the inner electrode.
Thus, the process of coating using CCP discharge proceeds at a very low rate and prolongs cycle time, which typically ranges from 10 to 15 seconds. Such a low duty cycle is not suitable for mass production of barrier-coated containers and limits throughput to six bottles per second. Furthermore, although high-power RF generators are expensive devices, in the case of CCP discharge they are used with low efficiency. For example, a valuable part of RF power is wasted for heating the outer and inner electrodes and for a parasite discharge in the space between the outer electrode and the outer wall of the container. A lengthy coating process can lead to melting of the containers, taking into account that the walls are heated by plasma. They are heated also from both sides by infrared irradiation emitted from the overheated inner and outer electrodes. Another problem associated with the use of CCP discharge is bias current driven by alternating voltage through the plastic. Such current creates additional heat, which deteriorates and melts the structure of the plastic walls.
Another obstacle is a high surface charge on the outer and inner surfaces of the walls that occurs between outer and inner discharges. UV radiation from plasma initiates photoemission from dielectric material that generates high electrical charge on the surface, and this, in turn, causes microarcs that destroy integrity of the thin film.
Another obstacle is a high-potential charge that remains on the surface of the container after deposition; this charge attracts dust, and therefore the container may require an additional sterilization.
Evacuation of containers at a high rate by means of a vacuum system for a quick drop in pressure is needed to create balance between high pressure inside the container and low pressure outside the container in order to reduce parasitic discharge, tight enveloping of the container for reducing the space between the outer wall of container and outer electrode with subsequent decrease of time needed for loading the containers, heating of both electrodes and plastic between them, and collapsing and charging of the container walls, all of which make the CCP discharge process highly inefficient in the formation of barrier coatings. Provision of the outer electrode makes it impossible to apply the coating onto the inner surfaces of plastic tanks and pipes having a curvilinear shape.
On the other hand, known in the art is ICP discharge, which is used as source of light and has been used as a source of light for some time. An ICP discharge has been described and analyzed in literature, such as in articles by R. B. Piejack, V. A. Godyak, and B. M. Alexandrovich titled “A simple analysis of an inductive RF discharge,” Plasma Sources Sci. Technol. 1, 1992, pages 179 to 186, and “Electrical and Light Characteristics of RF-Inductive Fluorescent Lamps,” Journal of the Illuminating Engineering Society, Winter 1994, pages 40 to 44. An ICP light source comprises a vacuum vessel, an inductive coupling system immersed in the vessel, and a high-frequency power source. In the initial stage of operation of inductively coupled plasma, an electrical field (E field) ionizes the fill in the gas-filled volume, and the discharge is initially a characteristic of an E discharge. Once breakdown occurs, however, an abrupt and visible transition to the H discharge occurs. Inductively coupled plasma works on the principal of producing an electric field in a body of gas by means of electromagnetic fields induced by oscillating current in the vicinity of the gas.
When the fields induced in the gas are strong enough, the gas can break down and become ionized in order to generate plasma. Such plasma has been used for a number of applications ranging from fluorescent lighting to plasma treatment of semiconductor wafers. During operation of an inductively coupled discharge, both E and H discharge components are present, but the applied H discharge component provides greater (usually much greater) power to the plasma than the applied E discharge component.
The inductively coupled plasma has been created by either wrapping a solenoid coil around a glass or quartz tube containing gas (“helical induction”) or by placing such a solenoid or spiral within the volume of gas itself (“immersed induction”). In a typical approach, an RLC circuit created by the inductive coil and a matching circuit are tuned to resonance and develop high currents on the coil. An alternating electromagnetic field induced within the gas volume creates a conductive plasma discharge having characteristics similar to secondary winding of a transformer, with a portion of the current through the discharge being converted to light. Lighting devices using immersed induction are described by Hewitt in U.S. Pat. No. 966,204, issued Aug. 2, 1910. Generation of light requires high plasma density in the center of a vessel so that the flat spirals, or solenoids, are immersed in a vacuum bulb having axial symmetry. However, use of axially symmetric antennas is not applicable to elongated containers, e.g., bottles, since they cannot generate plasma having high and uniform density near the inner walls of containers.
An example for use of capacitively coupled plasma for deposition of a barrier coating layer onto inner surfaces of bottles is disclosed in German Patent DE 3,908,418, by H. Grunwald, issued Sep. 20, 1990. This patent describes a system designed for plasma-assisted film deposition or treatment of hollow containers and comprises a capacitively coupled plasma system to drive a low-pressure gas discharge within the form. Such a system also has disadvantages, including a potentially lower deposition and treatment rate for mass-produced applications. Similar to other capacitively coupled plasma systems, the system of the aforementioned invention uses high plasma sheath energies that may result in excess heating of sensitive plastic container walls resulting in container damage. This design is also complicated and may require expensive and regular maintenance caused by film deposition on power-coupling components.
Also known in the art is the use of apparatus for coating the inner walls of containers, such as bottles, by means of deposition from inductively coupled plasma (see, e.g., U.S. Pat. No. 5,521,351 issued in 1996 to L. Mahoney). This invention relates to inductively coupled plasma generated within the interior of a hollow form held within a vacuum chamber enclosure by using a radio frequency coil mounted within the vacuum chamber around the outer surface of the container and closely conforming to the shape of the hollow container. The interior of a hollow form having complex shapes can be treated using two or more coils arranged to treat distinct portions of the form, and the shape of the coils and the manner in which power is supplied to the coils can be selected to control spatial distribution of the plasma within the hollow form.
A drawback of this system is low throughput because of non-optimal direction of the magnetic field generated by the coil. RF power applied to this coil provides RF current that generates an axial magnetic field. Therefore, plasma density in such a system is distributed so that maximum plasma density is concentrated in the vicinity of the axis but minimum plasma density is close to the inner wall of the container. Thus, coating of the walls in such a system has a low throughput rate.
The disadvantage of all apparatuses and methods for application of coatings onto the inner surfaces of containers known to the inventor is that some of the known methods and apparatuses are not suitable for application of silicone dioxide layers onto the inner surfaces of containers. Other conventional methods and apparatuses are difficult to control and do not allow deposition of thin coatings onto low-melting-point plastics at relatively low temperatures, and, most importantly, all methods and apparatuses known to the inventor have low efficiency and do not provide sufficiently high throughput in mass production. Existing antennas of apparatus for treating inner surfaces of containers have a geometry that does not produce plasma fields that match the inner profiles of containers.